Refuse collection vehicles include front loading and side loading vehicles for collecting refuse in containers placed on the side of a street by home or business. Front loading refuse collection vehicles use a lifting arm mounted to move between a position above the cab of the vehicle and a position in front of the vehicle. The arm moves from the position above the cab to the position in front of the vehicle, picks up a refuse container and lifts the container over the top of the cab, dumping the container's contents into the vehicle's refuse storage body. Side loading refuse collection vehicles use a lifting arm mounted on the vehicle's chassis or the refuse storage body. In side loading refuse collection vehicles, from a start position adjacent the side of the truck refuse body, the arm is extended to reach a refuse container.
At the outer end of either the side-mounted or front-mounted lifting arm is an apparatus that, upon actuation when positioned adjacent the refuse container, holds the container during lifting and dumping of the container. A grabber mechanism is used with the side-mounted arm that grips the refuse container and a fork mechanism is used with the front-mounted lifting arm that engages the refuse container. Both the grabber and fork mechanisms typically require the holding apparatus to be moved from a non-engagement position to an engagement position.
In both type of vehicles, the arm raises the container above the refuse storage body and tilts or inverts it to dump its contents into an open hopper in the top of the vehicle's refuse storage body. The lifting arm while holding the empty refuse container is then moved downward to set the container on the street. Finally, the arm is returned to its start position. The refuse storage body includes a packer to compact the refuse inside the body and the body is mounted to be tilted by a hoist to allow collected refuse to be discharged through an open rear door from the rear of the body. The rear door is closed while the packer is being operated and is opened to discharge the compacted refuse.
Different drive mechanisms have been used to operate the lifting arm, grabber mechanism, packer, the holding apparatus, and refuse storage body, for example, hydraulic cylinders, hydraulic motors, and hydro-mechanical actuators. Most of the refuse vehicle manufacturing industry uses gear pumps to operate these drive mechanisms. Such gear pumps are fixed displacement, low volumetric efficiency devices that cannot vary the flow rate from the pump without changing speed (which cannot be changed for control of output). These pumps supply the output fluid to the open-centered directional hydraulic control valves that supply fluid to the desired functional cylinders, motors and/or actuators. As such these pumps are on-off systems where, if any function is activated, all the fluid that is not used by the function is raised to a relief pressure setting and spilled over a relief valve, converting energy into heat. At the same time the pump's enormous leakage (which increases as the pressure and/or temperature rises) generates more heat and less useful work. Even when no function is operated and the fluid just flows through a valve, high flows cause large pressure drops that result in loss of energy into heat. To save energy at higher engine revolutions, when the pump operation is not needed, devices such as “dry valve” have been used to choke off the pump's inlet to stop the pump from pumping fluid.
To increase the volumetric efficiency some manufacturers have used vane pumps. Vane pumps have a much higher volumetric efficiency and thus reduce the energy losses due to pump leakage. Nevertheless, all the other losses stay the same as in the gear pump. Another disadvantage that vane pumps have is that unlike gear pumps, the flow from cannot be cut off. The vane pump has to pump out fluid when it turns. To reduce the pressure drop that would take place when pumping all the fluid through a directional control valve, especially at higher engine/pump speeds, a by-pass valve is used that opens when a signal is supplied to it to connect the discharge of the pump to suction, thus looping the fluid. This lowers the losses comparative to pumping through the valve, but a considerable pressure drop across the bypass valve and line occurs at higher speeds and results in major energy loss. To avoid raising all the fluid to the relief valve pressure, some manufacturers have used load sensing relief valves (unloading valves) to lower the relief pressure to the functional requirement. However, this requires load sensing directional control valves or other additional components. In spite of all of these energy loss reduction efforts, the losses remain extremely high in these pump systems, resulting in very high operating temperatures (175–185° F.). This in turn results in even higher losses due to loss of viscosity, higher component wear, higher seal failures, and hydraulic fluid oxidation requiring fluid changes.
U.S. Pat. No. 6,312,209 discloses a front loading vehicle using a variable displacement piston pump supplying fluid to closed-centered valves to actuate drive mechanisms. Front loading refuse collection vehicles are simple, slow moving systems that do not require the dynamics and controls necessary for side loading vehicles. This vehicle employs directional control valves that cannot accurately and repeatably control the speed of the lifting arm, since flow in these valves cannot be precisely adjusted. The reason for this inaccuracy and non-repeatability is pneumatic activation of the direction control valves. Besides the inaccurate and non-repeatable control achieved by a compressible pneumatic media, the system air pressure is not constant and results in different lifting arm speeds at the same control setting.